


Adaptive Immunity

by captainshellhead, vibraniumstark



Category: Marvel (Comics), Marvel 616, New Avengers (Comics)
Genre: Established Relationship, Extremis, Hurt/Comfort, M/M, Sickfic
Language: English
Status: Completed
Published: 2016-12-19
Updated: 2016-12-19
Packaged: 2018-09-09 16:55:25
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,867
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8900344
Author URL: https://archiveofourown.org/users/captainshellhead/pseuds/captainshellhead, https://archiveofourown.org/users/vibraniumstark/pseuds/vibraniumstark
Summary: Tony programs Extremis to act as his immune system, and it works a little too well.





	

**Author's Note:**

  * For [Veldeia](https://archiveofourown.org/users/Veldeia/gifts).



_Tony stepped into the partition and the pressurized door sealed behind him. The UV lights lining the walls snapped on at the same time as the modified fume hood above him whirred to life. It took nineteen seconds for the air to entirely circulate through the filtration system he had pieced together. He watched the time tick down on the armor’s HUD._

_Knowing that the lab was effective sterile made it seem more lonely. Or perhaps that was the large sheets of plastic he had over every wall, the low hum of air circulating, or the fact that he hadn’t spoken to anyone face to face since... Well, since he got sick._

_The unusual decor was a painful reminder. Here was was, the man in the bubble._

_It was a step up from being trapped in the armor, but only just._

_Tony walked over to his computer where he had left his latest--his only--project waiting for him. He would pick through every piece of code in Extremis if he had to. The screen blinked accusatorily at him. Three missed calls, all from Steve._

_Tony closed the notification. He had work to do._

 

 

Tony knew that off-white ceiling anywhere. 

He groaned and closed his eyes. His skin felt hot and tight, and the light stabbed at the back of his eyes like needles. 

“Steve,” he said. It was just a guess, but the sound of a chair scraping against the ground confirmed he was there. 

“Tony,” Steve said, sounding tired, sounding _frustrated_ , although Tony couldn’t imagine why. He couldn’t remember anything. He remembered the fight fairly well, but it had been almost over, and obviously something had happened, but Steve wasn’t the type to blame Tony for landing in the hospital after a mission unless he genuinely thought that it was Tony’s fault. 

“How are you feeling?” Steve asked. 

“Confused, mostly,” Tony said honestly. “I don’t remember what happened.”

Steve looked surprised to hear it. 

“We were fighting the wrecking crew. In midtown?”

“No, no. I remember the fight but... “ He shook his head slightly, but the motion was enough to send his vision spinning for a moment. He remembered the getting the call that morning. His head had been foggy, a slight pressure behind the eyes. Tony had assumed it was overstimulation. Extremis had opened him up to a whole world of stimuli that he had never even considered before, and even with what was essentially a computer for a brain, it was probably going to wear on him for a while to process it all. 

That didn’t explain how he’d ended up here. Last thing Tony remembered, they had been starting on _cleanup_ , the fight already over. “Did something hit me?” 

It was the wrong thing to ask, apparently. Steve’s lips curved into a deep frown.

“No, Tony. You fainted,” Steve said. Tony scoffed, but Steve ignored him. “The doctors ran some tests, but they couldn’t say what happened.”

Tony hummed. “I feel fine, now.” 

“I’m glad,” Steve said. “I think you should take some time off from the team.”

Tony sat up fast enough to spur another headrush. “What? Are you kidding me?”

“You fell two stories, Tony. If the armor’s failsafe hadn’t kicked in, it would have been twenty.”

“I’m sorry! How is that any different than _any_ other battle?” Tony said. 

“This wasn’t some act of villainy, Tony,” Steve said. “Pepper said you skipped a meeting this morning? You weren’t feeling well?”

“I was _fine_ , Steve. I have Extremis now! I can’t _get_ sick anymore!”

“That’s part of what I’m worried about,” Steve said. “You were beaten nearly to death, and then shot up with a super soldier serum we don’t know anything about, all less than a week ago. It’s not hard to figure out what’s going on.”

When Tony finally worked up the will to look over, Steve was wearing a frown that left a deep line in his forehead. As soon as they made eye contact, Steve spoke, as though he’d been waiting for Tony’s full attention. 

“You pushed yourself too hard,” Steve said. “It happens. I’m just asking that you take a little time off, until you’re back to yourself.”

Tony wasn’t ever going to _be_ back to his usual self. Extremis had changed him, there was no going back from that, and Tony knew that Steve wasn’t a fan of the idea. He kept that to himself. Nodded instead. 

“Fine. Have they said when I can go home?”

Steve seemed reluctant to answer. “They didn’t find anything wrong, but I think it would be better to wait for the labs to come back--”

“They can call me with the results,” Tony said sharply. He kicked the blankets off, and this time when he went to stand, his head didn’t spin. He was exhausted, still felt a little under the weather, but it must have been the aftermath of the fight, and the fact that he hadn’t eaten since this morning. Extremis had to burn more calories than usual, and it would take awhile for him to get used to the change. Maybe that was what had happened, earlier.

“There’s no point staying here if there’s nothing wrong with me,” he said. “I can rest at home just as easily as I can rest here.”

 

 

 

The pinging alert from his email pulled him half from his sleep to inform him that his doctor had updated his chart with his lab results. Seconds later, a call from the doctor followed, trilling at the back of his mind. He ignored it, let them go to voicemail, and accessed the lab results instead. 

He splayed the results on the back of his eyelids and let the voicemail play. 

_Mr. Stark, we need you to come back to the hospital immediately_.

Not likely. He skimmed through the numbers, looking for anything out of the ordinary. Or, well. Out of the ordinary for him, at least. 

_According to your lab results you’re extremely immunocompromised. You need to be quarantined at the very least. I have no idea how you were deemed healthy enough to leave—_

None of this was news to him. In order to keep his body from rejecting the Extremis modifications, he’d needed to make sure that his immune system didn’t outright reject it. The original programming had called for marking the Extremis as a part of himself, but that left the added problem of syncing Extremis with his own immune system. He hadn’t had the time for it, not with rampaging murderers on the loose. So he’d worked around it. Wiped his immune system clean and left the immune responsibilities to Extremis alone.

Why leave his body’s immune system up to something as variable as genetics when he could program the perfect immune system?

_Please call me back immediately._

Tony deleted the message. He looked at the labs again. At the time, it had been the obvious choice. The time it would have taken to rewrite something as complex as the human adaptive immune system would have resulted in more deaths. Deaths he could prevent. 

When Tony scrolled down to the lab results, he paused. 

Rhinovirus. Just a cold. How had Extremis missed that? He pulled up the subfiles for Extremis’s immune response history, pulled up today’s logs. 

Okay, so Extremis hadn’t missed it. 

The opposite, actually. It logged the exact moment that Extremis had first registered contact with the virus at 9:56am. Shortly after the immune systems had had a huge spike in activity. He looked through the logs as the system went haywire, mounting an enormous response against the foreign virus. 

Tony opened his eyes and sat up abruptly, suddenly wide awake. 

Tony crossed to his lab computer and dropped into the chair. When he reached for the computer mouse, he hesitated, the image of millions virus and bacteria waiting for a point of entry gave him pause. He was going to have to fix this, and _fast_ , but in the meantime…

Tony pulled his hand back, tucked it into his sleeve. He reached out to Extremis instead, calling up the specifications for a cleanroom. His workshop was already fitted for most of the modifications, and he wasn’t going to be able to work until he made certain that the lab was a safe and sterile environment to do so. He sent out a mass order for all the equipment he was missing, same-day delivery, and at the same time sent out the signal to call for the armor. 

The suit settled around him was a welcome relief, as the built-in air filtration system put itself to work. It would be a temporary measure. Hopefully, the equipment Tony had ordered would already be unnecessary by the time it arrived. 

Tony pulled up the code for Extremis next and jumped down to the section for innate and adaptive immunity. Beside it, he pulled up the original, hoping to find an obvious solution for how to tweak the immune system so that it wouldn’t go haywire at the slightest cold, but wouldn’t be too weak to protect him against anything else that he encountered in daily life.

There _had_ to be a solution. He just had to find it. 

If he didn’t, his overactive immune system may kill him.

 

 

 

The process turned out to be...more complicated than he’d anticipated. Tony spent all morning and into the afternoon picking through Extremis’s code. The delivery men came and went without Tony having made an ounce of progress, and he spent the rest of the evening installing the pieces for his new clean room, moving anything unnecessary and possibly contaminated out into the hall, and scrubbing everything else down with the container of seventy percent ethanol that Hank McCoy had left in his lab to collect dust years ago.

By the time he was finished and able to strip out of the armor, Tony was exhausted, frustrated, and ready to give it another go. He hadn’t been able to find a solution this morning, but maybe that was because he was looking for a way to tweak the current system. Maybe the problem was that Extremis _wasn’t_ working properly at all.

Tony started a diagnostic on Extremis, crossed the room to the lab sink, a colossal trek. He considered the hot plate, considered the kettle in the sink, and stuck a mug of water in the microwave instead. Crossed back to his computer, hot water and tea bag in hand, and sat down. He rested his head against the cool workbench and waited for the fog to clear. When the computer chirped at him, he glanced up at the screen.

He blinked, squinted, and started the scan again.

Again, he waited, and again, the computer chirped at him, signaling the completion of the diagnostic. Nothing. Extremis was working perfectly normal. Exactly as programmed. Tony drummed his fingers on the desktop and hummed thoughtfully. The computer chirped at him again, incessantly. 

No, not the computer. His Identicard was paging him, the ringing echoed in the back of his mind. He picked the card up, turning it over in his hand, an unnecessary redundancy now. With Extremis he opened his comm link, patched it through his private line. 

“Cap?” Tony asked. 

“Not you Tony,” Steve responded almost immediately. “You rest. We have this handled.”

“I’m fine,” Tony said. 

“I’m sure you’re fighting fit,” Steve said. “But I’m still not changing my mind.”

Tony reached out with Extremis, feeling for communications, scanning social media, probing for police scanner frequencies, searching for the cause for the call to assemble. Extremis patched it together for him. A bank robbery, unidentified assailants, but definitely superpowered by the looks of the damage. Nothing the Avengers couldn’t handle...probably just a precaution to send the whole team along. No sense in risking collateral damage, not with the Avengers so newly re-formed and looking to prove to the public that they could be relied upon again.

“You know,” Tony said. “Last I checked we were co-leading the team. It’s not exactly your place to bench me.”

A long-suffering sigh through the comms. Tony tossed the card down on the desktop, already knowing what Steve would say, already knowing how he would answer.

“Tony, please,” Steve said. 

“I know,” he said. “Be safe.”

 

 

The pounding on the door had stopped. A quick check of the clock suggested a probable reason: it was nearing one am. Steve had probably gone to bed. 

Tony knew he was worried. He hadn’t come out of the lab except for in the armor in days. All of his free time had been spent on going over Extremis, searching for a solution that he was slowly coming to realize would be anything but simple. The new security on the lab was a necessity--Tony couldn’t let just anyone waltz inside, lest he risk a deadly case of the flu--but it probably appeared petty at best to the other Avengers.

Steve probably thought that he was angry at him. 

In reality, Tony was dreading the admission that something was more wrong than he had originally thought. That his initial scare wasn’t a simple case of exhaustion, but a much more serious problem. 

The fact that they couldn’t have that conversation face-to-face without Steve going through a surgeon’s level of sterile procedure wouldn’t make the conversation any easier. Tony wanted to say that he was clinging to the hope that he would be able to find a solution. 

Really, he was recoiling from the thought of acknowledging that there may not _be_ a solution, and that he might have to move from looking for solutions to looking for accommodations. That he would have to accept this as his new normal.

He considered the notification from Steve: three message that Tony hadn’t been able to work up the willpower to listen to. He probably hadn’t fallen asleep yet. It wouldn’t be too unreasonable to call Steve down to talk to time. 

He drummed his fingers on the desk, and then wirelessly connected to his phone. 

 

 

Linda Carter hadn’t exactly been _happy_ about the request for a two am house call, but whether it was out of duty or she had somehow sensed the seriousness of Tony’s request, she’d still agreed to the unconventional meeting. 

Tony unlocked the door for her without a word. He got a odd look and and some raised eyebrows when Tony asked her to scrub down after passing through the entryway, but she took it in stride. It wasn’t until she’d finished and they’d settled in that she started needling him for answers.

“Can I ask why you called me?” Night Nurse said as she attached the blood pressure cuff to his arm. 

“Because I’m sick,” Tony said. 

“Okay, but why did you call _me_?” she pressed.

“Because I needed your expertise,” Tony said, “and because discretion is sort of your thing.”

She hummed and peeled back the velcro on the cuff. “Your blood pressure--”

“Is 123 over 79,” Tony said. She squinted at him, wondering how he’d guessed. He shrugged. Extremis was programmed to monitor his vitals.

“Normal,” she said. “Your temperature is a little high, though.”

It was hovering just shy of 101 degrees. The number itched at the back of his mind, a reminder of his mistakes. He nodded. 

“I’ve got a cold,” he said, “I think. The flu, maybe.”

Night Nurse leaned forward in her chair, tilting his chin up with her fingers, pressing gently against his neck. “Okay,” she said. “Now, don’t get me wrong. I’m happy to help, but...usually when I get a house call this late at night, it’s for bullet wounds, not the sniffles.”

“I’ve also have no immune system left,” he said. She jerked her hands back.

“You’re joking,” she said.

“You’ve probably seen my new suit,” Tony said. “It came with upgrades. Extremis. Super soldier serum meets supercomputer. But in order to keep my body from rejecting it, I had to put all immune responsibilities on Extremis. Wipe the slate.” She looked horrified. “But Extremis is too efficient. It’s going haywire at every little infection, and at this rate…” He shrugged.

“You’ve given yourself an autoimmune disorder,” she said. 

“I need to fix this,” he said.

“Tony, this isn’t something you can just fix. If you did this to yourself, your adaptive immune system is _gone_.”

“Right,” he said. “So.”

“So re-write your code to delete whatever it is that’s acting up, and get yourself on the bone marrow transplant list,” she said. “And until then, try not to get sick.”

She grabbed her bag. “And one more thing,” she said. 

“Hm?”

“You haven’t told your team, have you?” she asked.

“That conversation would go well.”

“You’re sick. They can help you,” she said.

“ _Who’s_ sick?” Steve asked. They both jumped and turned. Steve was standing in the doorway, still in his suit, face smeared with dirt and plaster dust. Maybe it hadn’t been his bed that had called him away but a mission. He must have come straight back after the fact to try Tony again. If he’d had any chance of playing it off as a misunderstanding, the guilty look on Night Nurse’s face squashed it. 

Tony didn’t respond, just swiveled in his chair. “Cap. I didn’t hear you come in. _How_ did you get in?”

“The door was unlocked,” Steve said, and then, unwilling to let himself be distracted. “I thought you couldn’t get sick. You told me Extremis wouldn’t let you get sick.”

“Right. About that,” he glanced over at Night Nurse, his cue for her to run while she still could. He doubted Steve would take this well. No sense in her suffering Steve’s wrath with him.

She put a hand on Tony’s shoulder. 

“I’ll leave you two alone,” she said. “Tony, I’m going to make some calls and get you put on the bone marrow transplant list. I already know what your answer is going to be, but you might want to consider checking yourself into a hospital in the meantime.”

Steve gave Tony an alarmed look. 

“Thanks,” Tony said. “I’ll think about it.” She pursed her lips but stood to collect her things. After a moment, she turned to eye Steve and his disheveled state. 

“Why don’t you scrub up in the sink there,” she said. “Before you give Tony strep and kill him.”

When Steve was done, Linda had made herself scarce, and Steve had found himself a seat a good ways away from Tony, Tony sighed. 

“I was going to tell the team,” Tony said defensively, “I just… wanted a sure idea of what was going on, first,” he said. Steve nodded reluctantly, at least willing to be understanding, but Tony could see that he was worried. He didn’t think that anything he had to say next was going to ease that worry. 

“I’m not angry,” Steve said. “Well. Angry at myself, maybe…”

“Steve, don’t,” Tony said.

“Tell me what’s wrong?” Steve asked.

“Extremis isn’t… working as perfectly as I’d hoped. When I was installing Extremis, I had to make some shortcuts.” Steve’s frown turned deeply disapproving, struggling between his true feelings on the matter and wanting to be gentle with him, not yet sure how fragile he was. “One of the things that I did was put Extremis in charge of my personal immune system, but it’s doing it’s job _too_ well. Any little bug sends my immune system into overdrive.”

“Is that what,” he waved a hand at the doorway, the new locks, “all this is about? How long have you known about this?”

“Since the doctor called me, the day I was discharged,” Tony said. “I’ve been looking for solutions ever since.”

“Can’t you fix it?” Steve asked. “Re...reprogram it, somehow?”

“The only way to fix it would be a hard reset of Extremis,” Tony said. “Re-write the code completely, removing the parts instructing Extremis to take over for my immune system.”

“And the bone marrow transplant?” he asked. 

Tony sighed. “Extremis completely wiped my immune system. So I need to borrow someone else’s, I guess.”

The concern on Steve’s face was killing him. Steve clenched and unclenched his fist at his side, like he wasn’t sure what to do with his hands. Tony rolled his eyes.

“You can touch me, Steve,” Tony said, amused despite himself. “I’m not going to keel over on the spot.”

Steve hesitated another moment, before reaching out to give Tony’s hand a squeeze. 

“We’ll fix this, Tony,” he said. 

 

Despite Steve’s assurances, even Captain America could only do so much. Tony could tell that it was killing him that he couldn’t do anything, and even despite many doctors’ assurances that being a blood type match was no guarantee for being a match for bone marrow donation, or their insistence that it was extremely rare for there to be a match that wasn’t familial, Steve still seemed to blame himself personally when the doctors told him that he wasn’t a match.

In the end, it was _Logan_ , of all people, who arrived the following week with a solution, after countless tests from multiple Avengers friends and family without luck. Salvation came in the form of a tough looking teenager, no older than any of Peter’s students, who he’d brought from upstate. 

“It feels like cheating,” Tony admitted. He was staring at the ceiling, struggling with whether he wanted to feel relieved, guilty, or something else. Steve squeezed his hand and he decided solidly on _somthing else_ at the warm feeling that spread through his chest at the simple act. “Not everyone has a shapeshifter at their disposal for an easy match.”

He glanced over at the partition separating him from the next room over, where his own face was staring idly at a table propped on his knees, with an expression of teenage defiance that looked entirely ridiculous on a grown man’s face while the nurse beside her gave her a speech that Tony could tell from here was a condescending spiel on the do’s and don’t’s of bone marrow donation. 

Tony could see the tablet playing Glee episodes on repeat from here, and he was fairly certain he’d seen Peter snapping pictures. 

“Don’t think of it that way,” Steve said. “There’s nothing wrong with getting better.”

“I know that. Still.” 

“Tony,” Steve said, leaning forward with such an earnest expression that it had him worried, a little. He tried to shift up into a more dignified position, but Steve just laid a hand on his arm to still him. “I’m sorry.”

“Sorry?” Tony parroted. “What do you have to be sorry for?”

“I’m sorry I didn’t ask you what was wrong sooner,” he said. “Or maybe I’m sorry that we’re right back at our old ways of not communicating with each other. And after we said it would be different, this time, with the new team.”

“We don’t learn, do we?” Tony asked. Steve smiled, eyes flicking down to Tony’s lips, thoughtful. Instead he leaned forward and pressed a dry kiss to his forehead, warm, soft. 

“Get some rest,” Steve said, while Tony stared at him, wide-eyed. “I’ll be here when you wake up.”


End file.
